Re: st: RE: statalist-digest V4 #4588 - was Graphing

[Nick Cox <njcoxstata@gmail.com>]

Similarly, the difference between ranks and grades was addressed in my  
2004 paper cited in my earlier posting.
Nick

On 24 Jul 2012, at 13:22, David Hoaglin <dchoaglin@gmail.com> wrote:

> Allan,
>
> I resisted the temptation to mention a trellis arrangement, in part
> because the bars to be compared would not have a common baseline.
>
> Stacked bars usually fail because the bars are stacked.  I was
> referring to empirical evidence, rather than authority.  Bill
> Cleveland and colleagues conducted studies of people's ability to make
> accurate comparisons in various graphical-perception tasks.  They
> arrived at the following ordering, from most accurate to least
> accurate (Cleveland 1985, Table 4.3):
> 1. Position along a common scale
> 2. Position along identical, nonaligned scales
> 3. Length
> 4. Angle --- Slope
> 5. Area
> 6. Volume
> 7. Color hue --- Color saturation --- Density .
> In a stacked bar chart, the comparisons are among lengths (unless some
> variable is encoded in them, the widths carry no information).  Why
> not move up in the hierarchy when one can?
>
> I agree that it is important to distinguish data that are ranks from
> data that are grades.
>
> Also, as often, some analysis is likely to be helpful in understanding
> the patterns in the data and deciding what to present.
>
> David Hoaglin
>
> William S. Cleveland. The Elements of Graphing Data. Wadsworth
> Advanced Books and Software, 1985.
>
> W. S. Cleveland and R. McGill. Graphical perception: theory,
> experimentation, and application to the development of graphical
> methods.  Journal of the American Statistical Association 1984;
> 79:531-554.
>
> W. S. Cleveland and R. McGill. Graphical perception and graphical
> methods for analyzing scientific data.  Science 1985; 229:828-833.
>
> On Tue, Jul 24, 2012 at 6:22 AM, Allan Reese (Cefas)
> <allan.reese@cefas.co.uk> wrote:
> > The answers offered to Aminu may be helpful but they appear to ignore
> > the contradiction in the question as posed:
> > [Aminu] I have a qualitative data where 8 diseases ranked (1
> > (most-important) to 5 (least important)) based on perception - 37
> > subjects were interviewed so 37 records in the dataset.
> >
> > It appears the diseases were not *ranked* but *graded*, an important
> > distinction.  If ranked, only one disease can be first for each  
> > person,
> > and the ranks would run 1-8.  If graded, could one person think all
> > eight had the same importance?  Before offering code to draw specific
> > graphs, it is necessary to know the intended use of the graph  
> > (analysis
> > or presentation); if the latter, what message is it intended to  
> > convey?
> > For example, a slide to make the point "everyone thinks this  
> > disease is
> > important, but this one is considered trivial" might well use eight
> > stacked bars and vivid colours.
> >
> >
> > [David Hoaglin ] "A key message is that stacked bars are generally  
> > a bad
> > idea.  You would do better with a little histogram for each disease  
> > (5
> > bars, each sitting on the horizontal axis) and no numbers on top.
> >
> > "This approach has the advantage that the 8 bars for each rank,  
> > though
> > not adjacent, have a common baseline."
> >
> > [AR] 8 histograms side by side may be less easy to compare than 8
> > stacked vertically in a trellis. This is like comparing "profiles" in
> > correspondence analysis.  Rather than take authoritarian advice,  
> > Aminu
> > might try both and see which conveys the intended message.  This  
> > may be
> > culturally dependent (relating to direction of reading) and I think
> > Aminu in is Nigeria.
> >
> > [Nick Cox] My bottom line is that stacking is often chosen, but  
> > rarely
> > optimal. One disadvantage of stacking is that it imposes dependence  
> > on a
> > key or legend.
> >
> > [AR] I'll agree with Nick that random choices from a "graphics  
> > gallery"
> > are often confused and confusing. Stacked bars often fail because the
> > author didn't think about the order of stacking - Excel stacks  
> > values in
> > alphabetic order of labels.  Stata makes it easy to choose the  
> > order and
> > here the values are inherently ordered, so reference to a legend is  
> > less
> > problematic. Since the values for each reply are discrete (no of  
> > 1s, no
> > of 2s etc), another option is a line chart for each disease, so the
> > slope between values helps the visual comparison of "profiles".
> >
> > I'll guess the data were input as 37 cases with a variable for the
> > importance of each disease.  If -reshaped- as 37*8 cases with
> > subject/disease/importance as variables, -tab disease importance-  
> > gives
> > the table of counts that the graph is intended to illustrate.   
> > Hence one
> > can do some analysis before drawing a graph to present the findings.
> >
> > Allan
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